The question as to the nature of light rays is one of the oldest
problems in physics. In the works of the ancient philosophers are
to be found an indication and a rough outline of two radically
different concepts of this phenomenon. However, in a clear and
definite form they appear at the time when the foundations of
physics were laid, a time that bears the stamp of Newton's
genius. One of these theories asserts that a light ray is
composed of small particles, which we may term corpuscles, which
are projected into space by light-emitting substances. The other
states that light is a wave motion of one type or another. The
fact that these two theories, at this elementary stage, are
equally possible, is attributable to their explaining equally
well the simplest law governing a light ray, viz. conditions
being undisturbed it propagates in a straight line.

The 19th century sealed the victory of the wave theory. Those of
us whose studies coincide with that period have certainly all
learned that light is a wave motion. This conviction was based on
the study of a series of phenomena which are readily accounted
for by the wave theory but which, on the other hand, cannot be
explained by the corpuscular theory. One of these phenomena is
the diffraction undergone by a light beam when it passes through
a small hole in an opaque screen. Alongside the diffracted ray
there are alternate light and dark bands. This phenomenon has
long been considered a decisive proof of the wave theory.
Furthermore, in the course of the 19th century a very large
number of other, more complex, light phenomena had been learnt of
which all, without exception, were completely explainable by the
wave theory, while it appeared to be impossible to account for
them on the basis of the corpuscular theory. The correctness of
the wave theory seemed definitely established.

The 19th century was also the period when atomic concepts have
taken root into physics. One of the greatest discoveries of the
final decades of that century was the discovery of the electron,
the smallest negative charge of electricity occurring in the free
state.

Under the influence of these two currents of ideas the concept
which 19th century physics had of the universe was the following.
The universe was divided into two smaller worlds. One was the
world of light, of waves; the other was the world of matter, of
atoms and electrons. The perceptible appearance of the universe
was conditioned by the interaction of these two worlds.

Our century taught us that besides the innumerable light
phenomena which testify to the truth of the wave theory, there
are others which testify no less decisively to the correctness of
the corpuscular theory. A light ray has the property of
liberating a stream of electrons from a substance. The number of
electrons liberated depends on the intensity of the ray. But the
velocity with which the electrons leave the substance is the same
whether the light ray originates from the most powerful light
source that can be made, or whether it originates from the most
distant fixed stars which are invisible to the naked eye. In this
case everything occurs as if the light ray were composed of
corpuscles which traversed the spaces of the universe unmodified.
It thus seems that light is at once a wave motion and a stream of
corpuscles. Some of its properties are explained by the former
supposition, others by the second. Both must be true.

Louis de Broglie had the boldness to maintain that not all the
properties of matter can be explained by the theory that it
consists of corpuscles. Apart from the numberless phenomena which
can be accounted for by this theory, there are others, according
to him, which can be explained only by assuming that matter is,
by its nature, a wave motion. At a time when no single known fact
supported this theory, Louis de Broglie asserted that a stream of
electrons which passed through a very small hole in an opaque
screen must exhibit the same phenomena as a light ray under the
same conditions. It was not quite in this way that Louis de
Broglie's experimental investigation concerning his theory took
place. Instead, the phenomena arising when beams of electrons are
reflected by crystalline surfaces, or when they penetrate thin
sheets, etc. were turned to account. The experimental results
obtained by these various methods have fully substantiated Louis
de Broglie's theory. It is thus a fact that matter has properties
which can be interpreted only by assuming that matter is of a
wave nature. An aspect of the nature of matter which is
completely new and previously quite unsuspected has thus been
revealed to us.

Hence there are not two worlds, one of light and waves, one of
matter and corpuscles. There is only a single universe. Some of
its properties can be accounted for by the wave theory, others by
the corpuscular theory.

In conclusion I would like to point out that what applies to
matter applies also to ourselves since, from a certain point of
view, we are part of matter.

A well-known Swedish poem has as its opening words "My life is a
wave". The poet could also have expressed his thought by the
words: "I am a wave". Had he done so, his words would have
contained a premonition of man's present deepest understanding of
the nature of matter.

Monsieur Louis de Broglie. When quite young
you threw yourself into the controversy raging round the most
profound problem in physics. You had the boldness to assert,
without the support of any known fact, that matter had not only a
corpuscular nature, but also a wave nature. Experiment came later
and established the correctness of your view. You have covered in
fresh glory a name already crowned for centuries with honour. The
Royal Academy of Sciences has sought to reward your discovery
with the highest recompense of which it is capable. I would ask
you to receive from the hands of our King the Nobel Physics Prize
for 1929.